Downgrade Ios 9.3.5 To Ios 8.4.1 Link -

idevicerestore.exe -w [8.4.1.ipsw]

idevicerestore -w custom_8.4.1.ipsw

Connect your device to iTunes and perform a full backup. Remember, you cannot restore this backup once on iOS 8.4.1, but it is essential to have it in case something goes wrong and you need to revert to iOS 9.3.5. downgrade ios 9.3.5 to ios 8.4.1

Now that you have a custom IPSW built, you need to put your device into a special "pwned" DFU mode so it accepts this custom firmware. idevicerestore